Brakes designed for the purpose of immobilizing a parked vehicle usually wear very rapidly if the brake is engaged during powered travel of the vehicle. In the absence of protective interlock systems, unnecessary brake wear of this kind most commonly occurs at start-up as operators may on occasion forget to release the brake when powered travel is being initiated.
A variety of interlock systems have heretofore been used to prevent "drive through" or start-ups of a vehicle with the parking brake engaged. Prior interlocks variously employ electromechanical, fluidic or mechanical linkage means to sense the condition of the brake and to actuate a means for blocking the flow of power between the engine and the drive wheels or the like while the brake remains engaged. More specifically, prior interlocks typically act to maintain or establish a neutral condition in the vehicle transmission until the parking brake is released.
While being effective for the basic purpose of preventing drive through, such prior interlocks have the further effect of restricting certain modes of vehicle operation which can be desirable under some conditions.
Automatic neutralization of the transmission when the brake is engaged may, for example, complicate the start-up process if the vehicle is on a slope or under heavy load. In some cases, interlocks which hold the transmission in neutral if the brake is engaged do not allow the engine to be accelerated with the transmission already in gear, prior to release of the parking brake, as is desirable to facilitate start-up under such conditions. Further, while it is not desirable to use the parking brake to slow or stop the traveling vehicle under most conditions, this can become necessary on occasion because of malfunction of the primary or service brakes. Where the parking brake is used in an emergency situation of this kind, many of the above discussed prior interlock systems force the transmission into neutral simultaneously with application of the parking brake. Consequently, the retarding action or dynamic braking effect of the engine is not available to supplement the parking brake in slowing or stopping the vehicle.
Depending on the nature of the specific interlock and vehicle drive system, there are often still other effects which tend to limit operational flexibility of the vehicle under various different operating conditions.
Certain problems involved in using known forms of interlock system tend to be most pronounced in vehicles which employ a so-called mechanical or manually shifted gear box type of transmission connected in tandem with a manually operated drive clutch. Forcing of a neutral condition, when the parking brake is engaged, is a relatively simple matter in transmissions of the fluid pressure controlled or power shift type which includes many planetary transmissions. These are commonly provided with a neutralizing signal port and the interlock system may simply transmit a pressure signal to the port to achieve the desired neutral condition.
This is not possible in most mechanical transmissions. Where prior brake interlock systems are used with manual transmissions, it has been necessary to provide complex mechanical linkages to lock the operator's shift lever in a neutral setting or in some cases to provide power actuators to force the shift lever or linkage back to the neutral position in opposition to the operator's efforts.
In general prior brake interlock systems accomplish the desired objective of preventing drive through only at the cost of introducing other less objectionable complications into the structure or operation of the vehicle or both.